The length of the string between the nocks (or from where it touches either limb on a recurve) divided by 6 and 10. For example, say the string is 60 inches long. The silencers would be placed six and 10 inches from the ends. That's a lot closer to the ends than most folks put silencers.
The other harmonic dampening formula says to place them at 1/4 and 1/3 the string length. That would be 15 and 20 inches from the ends on a 60-inch string.
Some folks just put their silencers a given distance from the string ends. For example, I used to put mine about 12 inches from the ends, regardless of string length.
They all silence the string some, but so far, the L6 and L10 positions seem to work the best for me, and they're pretty much out of the way of everything else.